Origin of Life: I Monomers to Polymers - Astronomy Program - The

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Origin of Life: IMonomers to Polymers

Synthesis of Monomers

Life arose early on Earth (within 0.7 ¥ 109 y)

1. Conditions1. Liquid Water2. Reducing or Neutral Atmosphere3. Energy Sources

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2. Originally thought atmosphere wasNH3, CH4, H2O, H2

Miller -Urey Experiment

Now Believe CO2, H2O, N2

3. Energy Sources

Ultraviolet Light (No Ozone)LightningGeothermal (Lava, Hot Springs, Vents, …)

Miller -Urey Experiment

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Relative Yield27021

1454

64210.4

22

666

1331000

6456211786

COMPOUNDGlycineSarcosineAlanineN-methylalanineBeta-alanineAlpha-amino-n-butyric acidAlpha-aminoisobutyric acidAspartic acidGlutamic acidIminodiacetic acidIminoacetic0propionic acidLactic acidFormic acidAcetic acidPropionic acidAlpha-hydroxybutyric acidSuccinic acidUreaN-methyl urea

How did Amino Acids form in Miller -Urey Experiment?

Strecker Synthesis

CH4, H2, NH3 + Energy H2CO, HCN, HC3N,e.g. Glycine Synthesis Urea (H2 NCONH2)

Reactive

H2CO + NH3 + HCN N C C ≡ N + H2O

H

H

H

HAminoacetonitrile

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N C C ≡ N + H2O + H2O

H

H

H

H

N C C + NH3

H

H

H

H

O

O H

glycine

H2CO C = O form AldehydeH

HMore complex group - other aldehydes

more complex amino acids

Lower yield if atmosphere was N2, CO2, H2O(If H2/CO2 > 2, get good yield)

Problems with Miller - Urey

Atmosphere was N2, CO2, H2O

NH3, CH4 would react N2, CO2

Try N2, CO2, H2O in Miller - Urey simulation

Only get trace amounts of glycineNeed CH4 to get more complex amino acids

Need H2/CO2 > 2 to get much of any amino acid

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Miller - Urey with Cosmic Rays

A group in Japan has obtained good yields ofamino acids from slightly reducing gases

(CO2, CO, N2, H2O)

When they used high energy protons(simulate cosmic rays)

Apparently not Strecker Synthesis(Low abundance of aminoacetonitrile)

Building Blocks of Nucleic Acids

Not formed in Miller - Urey But some intermediates were1. Ribose Sugar:

5 • H2CO + Heat H10C5O5 [Clay Catalyst]

2. Basesa) Purines 5 HCN H5C5N5

(Adenine)

b) Pyrimidines HC3N + Urea H5C4N3O

(Cytosine)

(1995) Cyanoacetaldehyde + Urea Uracil

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3. PhosphateRock Erosion

Less understood than amino acids

Other Possibilities:Seafloor VentsInterstellar MoleculesComets

Alternative Delivery Molecular clouds - strongly reducing, contain many

molecules used in Miller-Urey (H2, NH3, H2O, CH4)and intermediates (HCN, H2CO, HC3N) and possiblyglycine

Problem: These would not have survived in part ofdisk where Earth formed

But interstellar ices comets Evidence from similar molecules (e.g. C2H2, CH4, HNC, …) Clearly indicates interstellar chemistry

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Cratering record on moon, …fi heavy bombardment early in history

Comets and their debris could have broughtlarge amounts of “organic” matter to Earth

(and maybe oceans)

Some evidence for non-biological amino acidsin layer depostied after asteroid impact 65million years ago

Sources of Organic MoleculesQuantitative comparison by Chyba & Sagan, Nature

1992, Vol. 355, p. 125

Currently, Earth accretes ~ 3.2 ¥ 106 kg y-1 frominterplanetary dust particles (IDP)

~ 10 % organic carbon fi 3.2 ¥ 105 kg y-1

~ 103 kg y-1 comets~ 10 kg y-1 meteorites

~ 103 ¥ more at 4.5 ¥ 109 yr ago (?)(cratering record)

UV + reducing atmosphere 2 ¥ 1011 kg y-1

But if H2/CO < 0.1 IDP’s dominant source~

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So if atmosphere very neutral, IDP’sMost of mass in IDP’s in range of size ~ 100 mmmass ~ 10-5 gComplex structure - composites of smaller grains

some carbon richEnhanced deuterium low T

Also found in interstellar molecules? fi connection back to interstellar chemistry

2 kinds(mass ranges) can supplyorganic matter

1.Interplanetary dust particles (m < 10-5 g)

2.Smaller meteorites (m < 108 g)

~

~

Mas

s Ac

cret

ion

Rate

on

Earth

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Alternative Sites

Locally reducing environments

1. Ocean ventsSources of CH4 and H2SCurrent Vents have ecosystems based onenergy from chemicals - not photosynthesisH2S Bacteria Clams, Tube WormsPre-biotic amino acid synthesis?

2. Inside EarthMany bacteria now known to live deep(~ 2 miles) in Earth. Again, on chemicals,adapted to high temperature genetic makeupvery ancient

3. Hot SpringsBacteria may be important in precipitatingminerals. again adapted to high T and

ancient

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Synthesis of PolymersM1 + M2 P + H2O more likely in liquid H2O

Solutions Remove H2O (Drying, Heat)Sydney Fox Proteinoids

Energy Releasing Reactions (H2NCN or HC3N) Catalysts: Clays

Monomers polymers

A problem:Peptide bond requires removal of H2OThis would be hard in primordial sea

Need special molecules to do what Ribosome does inliving cells

Input of energyor

Dry environment (dry land)

Imagine drying tidepool + geothermal heatHeat + amino acids peptide bondsSidney Fox “proteinoids”or catalyst - clay, energy-rich bonds…

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Problem greater for nucleic acidsSugar + base + heat some nucleosides

Activated nucleosides + phosphoric acid + Zn+2

polymers up to 50 nucleotides

linkages (mostly) correct

nucleoside nucleotide

Nucleic acids more complex

Monomers of nucleic acids

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Synthesis of Adenosine

Base on 1’ Carbon Why?

Adenine + ribose sugar adenosine + H2O

Also phosphates 3’ & 5’ carbons

Otherwise

Leslie Orgel has had some success ingetting high percentage of correct linkages,in presence of Zinc ions.

Misalignment

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The Odds

• We need to get an “interesting” polymer– Enzyme– Self replicator

• Properties of polymer depend on– Order in which monomers combine

• If we combine monomers at random,– How likely to get something interesting?

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Statistics of an unlikely eventRandom reactions in primordial soup?Unlikely event versus many trialsProbability Consider tossing 10 coins

Probability of all heads = product of prob.P = 1 1 1 1 1

2 2 2 2 2 ( )( )( )( )( ) … ( ) 12

10 = 11024

Probability of getting 10 amino acids proteinChosen from 20 in a particular order

120( )10 = 1

1 ¥ 1013

But if you try many times, the chance ofsuccess is higherP(r) = n!

r! (n - r)! pr (1 - p)n-r

r = # of successes p = prob. Of success on each trialn = # of trialsn! = n (n-1) (n-2) … 1e.g. make n = (flip all 10 coins 1024 times)1

pP(1) = n!

1! (n - 1)!1n ( )( ) 1- 1

nn - 1

= 0.37

Chance of one or more successes = 0.63For reasonable chance need n ~ 1

p

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How many do we have to get right?1. How many atoms?

Lipids 102 - 103

Enzymes, RNA 103 - 105

Bacterial DNA 108 - 109

Bacterium 1011 - 1012

Human Being 1027 - 1028

If we choose from H,C, N, O (ignore S,P)probability of right choice 1/4So for enzyme: ( )103 ~ 10-6001

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# of trials: Shapiro computesN = 2.5 ¥ 1051 (surely an overestimate) n << for simple enzyme

2. What if we start with amino acids?Need ~ 1013 trials to get 10 amino acid protein

To get 200 amino acids in right order

1p

120( )200 = 10-260 Hopeless!

Need something besides random combinationsSelection (Natural?)

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Improving the OddsMany proteins composed of interchangeablesegments (Domains)

10 -250 amino acids

One domain found in ~ 70 different proteins

Intermediate building blocks?

If so, may only need to get enough amino acidsin right order for a domain

e.g. 18 amino acid domain

P = 10-23

Also, many variations in amino acids don’tdestroy function

and many different sequences may beinteresting

120( )18

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Scientific American Doolittle & BorkOct. 1993, pg. 50

Proteins made of domains, assembled in various ways10-250 amino acids for ones containing disulfide bonds

18 - 100 for those without

Of all amino acids available

120( )40 1

20( )18or

log10 = 40 log 20 -18 log 20= - 52 = -23.4so 10-52 10-23.4

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Interesting fact on how the improbable happens1st winner of Texas Lotto lotteryPicked all 6 numbers correctly in the sameorder as they were drawn.

Each number runs from 1 to 50, and oncechosen, cannot be repeated (balls are takenfrom a box).

So the odds against getting them in order is

You don’t need to get them in the same order to win -odds against winning include any combination,so 1 in 16 million

150( ) 1

49148

147

146

145( )( )( )( )( ) = 1

11,441,304,000